1. Technical Field
The present invention relates to an Ethernet protection scheme which is implemented using p-cycle techniques in a connection-oriented Ethernet network.
2. Description of Related Art
Ethernet is an example of an OSI layer 2 communications protocol. Conventional Ethernet switching apparatus forwards received Ethernet frames on the basis of Media Access Control (MAC) address information. The frames are forwarded in a connectionless manner, i.e., the path each frame follows is not pre-configured and each frame is capable of being forwarded by switching apparatus independently along a different route towards the destination MAC address node.
Conventional Ethernet switching apparatus operates a broadcast-on-unknown mode of operation if a frame is received with destination address information for which no association with an outgoing interface (e.g., outgoing port) of the switching apparatus exists in the forwarding table of that switching apparatus. The broadcast-on-unknown (or flooding) process enables the switching apparatus to determine appropriate entries in its forwarding tables in the manner well known to those skilled in the art. However, to ensure that broadcast traffic does not unduly escalate whenever the physical topology of the Ethernet network contains fully connected nodes (which would allow looping to occur), protocols such as Spanning Tree are utilised by the switching apparatus to virtually reconfigure the outgoing ports of the switching apparatus to logically configure the switching nodes into a fully-connected tree configuration and thus eliminate any physical looped topologies.
It is known to adapt switching apparatus arranged to forward Ethernet frames using conventional techniques to support connection-oriented services by populating the switching apparatus directly from the control plane and discarding all frames with unknown destination address information in the header fields. More details on connection-oriented switching using Ethernet frames is known in the art from, for example, International Patent Application WO2006070197 entitled “CONNECTION-ORIENTED COMMUNICATIONS SCHEME FOR CONNECTION-LESS COMMUNICATIONS TRAFFIC” and United States Patent Application US 2005/0220096 entitled “TRAFFIC ENGINEERING IN FRAME-BASED CARRIER NETWORKS” by Friskney et al.
The invention assumes that looped topologies are retained between Ethernet switching apparatus so that one of the abovementioned known methods for reconfiguring the routing tables of Ethernet switching apparatus has already been implemented. This permits multiple paths to be provided for Ethernet traffic between source and destination nodes.
Protection switching is possible whenever multiple paths are possible between source and destination nodes. As is well known to those skilled in the art, protection switching requires an alternative path to be configured in the network which is utilised by traffic whenever a link or node failure on the original path occurs. One conventional protection switching scheme requires a complete alternative path to be pre-configured and for each node on the protection path to allocate appropriate resources so that in the event of the original path failing, traffic can be switched over as rapidly as possible. For example, some known protection schemes require each node on the protection path to reserve bandwidth to match the required path bandwidth. These are referred to in the art as 1+1 protection schemes and are limited in that twice the bandwidth needed by the traffic to be protected must be reserved.
Various schemes are known in the art to reduce the amount of bandwidth which is reserved by a protection scheme, including schemes which are implemented using p-cycles. In brief, a p-Cycle is a protection scheme that pre-reserves bandwidth in the communications network to provide path protection in the event of path failure. P-Cycles are known in the art for both connection-oriented and for connection-less traffic such as traffic using the IP transport communications protocol. Conventionally, p-cycle is configured by entering appropriate routing information into the forwarding table associated with the switching apparatus of a communications network. Once traffic is passed onto the p-cycle, traffic follows the forwarding table entries.
A p-cycle protection scheme does not associate the p-cycle with the working capacity of the network or limit how the protected path is routed. This allows the p-cycle to have any route through the network. Switching speed is maintained by the pre-connection feature of p-cycles as only the end-nodes adjacent to the failed link or node need to switch traffic to the p-cycle. A p-cycle scheme can be implemented locally, by nodes adjacent to the failure, on a time-scale of the order of a few milliseconds.
FIG. 1A shows an example of a communications network 10 in which a p-cycle such as is known in the art has been provisioned to protect communications traffic. In FIG. 1A, communications network 10 is schematically represented by a plurality of letter labelled nodes 12 and interconnecting physical communication links 14. As is well known to those of ordinary skill in the art, each pair of adjacent nodes 12 may be connected by one or more logical and/or physical communications channels along each physical communications link 14. The term link failure in the context considered herein refers to the failure of the physical link (affecting one or more channels along that link). Communications links 14 shown in FIG. 1A by the thick dashed line form part of a p-cycle 16. The basic properties of p-cycles are already known in the art and more details can be found in the following document, for example see: Network Protection with Ring-speed and Mesh-efficiency”, by Schupke, Gruper, Grover and Stamatelakis, COST 270 Workshop, 2001.
Other known protection schemes include ring protection schemes which reserve bandwidth and protect against link and node failures occurring on one of the nodes or links already forming the protection ring (see FIG. 1B). In contrast, p-cycles provide protection for link and node failures both on-ring and “off-ring” (see FIG. 1C for an example of an off-ring failure). “Off-ring” failures are referred to as “straddling failures” in the art.
Referring again to FIGS. 1A to 1C of the accompanying drawings, these show a p-Cycle comprising 9 protected spans “on-cycle”. FIG. 1B shows by way of example, how traffic originally going from node B to node D may be protected in the event that the communications link between nodes B and D fails (an on-ring failure). Once the failure has been detected by the node through which traffic passes prior to the failed link (here node B) using any appropriate technique already known to those of ordinary skill in the art, communications traffic at node B is mapped by the forwarding table for node B onto the p-cycle and is transported along the p-cycle pre-provisioned path from node B via nodes A, I, C, H, F, E, and C to finally end up at node D. At node D, the traffic leaves the p-cycle and is forwarded from node D based on the pre-failure entry in the forwarding table for D. In other words, once the traffic is restored to the node following the point of failure, it reverts back to its behaviour prior to encountering the failure.
FIG. 1C shows that the same p-cycle provides protection for 8 “straddling” communications links between non-adjacent nodes on the p-cycle. In FIG. 1C, the communications link between nodes C and I has failed. Traffic originally for routing between nodes I and C is instead routed along one or both of the available paths on the p-cycle via nodes IABDC and/or via nodes IGHFEC.
Additional background information on p-cycles can be found in W. D. Grover, D. Stamatelakis, “Bridging the ring-mesh dichotomy with p-cycles,” Proc. Second International Workshop on the Design of Reliable Communication Networks (DRCN 2000), Munich, Germany, 9-12 Apr. 2000, pp. 92-104. W. D. Grover, “Understanding p-cycles, enhanced rings, and oriented cycle covers,” Proc. First International Conference on Optical Communications and Networks (ICOCN 2002), Singapore, 11-14 Nov. 2002, pp. 305-308. and W. Grover, “p-Cycles, ring-mesh hybrids and ‘ring-mining:’ Options for new and evolving optical networks,” slides presented at Optical Fiber Communication Conference (OFC) 2003, Atlanta, Ga., USA, 24-27 Mar. 2003, “p-Cycles: Network Protection with Ring-Speed and Mesh-efficiency” by Schupke, Gruber, Grover and Stamatelakis, COST 270 Workshop 2001; Chapter 10 of “Mesh based Survivable Networks” by Grover, Prentice Hall 2004; and “Automatic Protection Switching for p-cycles in WDM networks” by D A Schupke, Optical Switching & Networking, Elsevier, April 2005.
Whilst p-cycle protection schemes are known in the art, it has hitherto not been possible to implement a p-cycle protection scheme for conventional Ethernet traffic as the broadcast-on-unknown mode of operation of conventional Ethernet switching has resulted in the need for protocols such as spanning tree to be implemented to explicitly prevent any multiple path configurations from being possible within an Ethernet network.
It is known in the art for Ethernet switches to switch off spanning tree so that they can be configured to route traffic to a destination address in a connection-oriented way, for example, based on performing a look-up of both a globally unique identifier such as the MAC destination address and another field identifier such as the VLAN identifier. The globally unique identifier enables Ethernet switching apparatus to switch over an inter-network comprising a plurality of local area networks, and the VLAN-identifier enables different paths to be pursued by Ethernet traffic with the same source and destination address. Other known types of connection oriented Ethernet forwarding include:                Destination Address based forwarding                    forwarding is based on DA, DA is globally unique                        MAC swapping                    forwarding is based on DA, DA value can change on link-by-link basis                        VLAN switching with global VLAN ID                    forwarding is based purely on the VLAN ID, VLAN ID same on all links                        VLAN swapping                    forwarding is based purely on the VLAN ID but value can change on link-by-link basis                        Provider Backbone Transport                    forwarding based on VLAN ID and DA—same values on all links.                        
For each of these connection-oriented Ethernet schemes that it is possible to implement p-cycle protection for Ethernet as the spanning tree protocol is switched off and population of the forwarding tables is done using management configuration options. In contrast to known connection-oriented p-cycle protection schemes, however, the large number of potential links and the volume of the address space of the router tables for Ethernet switching apparatus means that it is not possible to utilize the same approach for an Ethernet p-cycle protection scheme.
The invention relates to a scheme for implementing p-cycle protection for Ethernet traffic, which seeks to avoid the problems associated with the use of assigned or interchanged labels when seeking to implement a p-cycle protection scheme in an Ethernet network.